1. Field of the Invention
The present invention relates to thermoplastic resin particles for forming foam containers, such as cups, bowls, etc.; to a molded article, e.g. foam container made from the resin particles; to a coating composition for the thermoplastic resin particles; and to a method for improving the resistance to leakage of a foam container. More particularly, the present invention pertains to a coating composition for expandable or pre-expanded, i.e. “pre-puff”, thermoplastic resin particles, e.g. polystyrene, used in molding containers for carrying liquids, e.g. coffee or foods containing oil and/or fat components such as precooked (instant noodles, soups, sauces, fried chicken) fat-containing foods, and the like.
2. Background Art
The manufacture of molded articles, e.g. containers, e.g. cups, bowls, made from expandable thermoplastic particles is well known. The most commonly used thermoplastic particles are expandable polystyrene particles, referred to as “EPS” particles. Typically, polystyrene beads are impregnated with a blowing agent, which boils below the softening point of the polystyrene and causes the impregnated beads to expand when they are heated. When the pre-expanded beads are heated in a mold cavity, they further expand to fill the cavity and fuse together to form a shaped article, e.g. containers, e.g. cups, bowls, etc.
The formation of molded articles from impregnated polystyrene beads is generally done in two steps. First, the impregnated polystyrene beads are pre-expanded to a density of from about 2 to 12 pounds per cubic foot. The pre-expanded beads are typically called “pre-puff”. This first step, i.e. the pre-expansion step, is conventionally carried out by heating the impregnated beads using any conventional heating medium such as steam, hot air, hot water, or radiant heat. Second, the pre-expanded beads (“pre-puff”) are heated in a closed mold to further expand the pre-expanded beads to form a fused article having the shape of the mold. This latter step is generally referred to as molding.
The expandable polystyrene particles used to make foam containers are generally prepared by an aqueous suspension polymerization process, which results in beads that can be screened to relatively precise bead sizes. Typically, bead diameters are within the range of from about 0.008 to about 0.02 inch. Occasionally, cups are made from particles having bead diameters as much as 0.03 inches.
In spite of careful bead size control, one problem, which continues to plague the container, industry is that after a period of time the EPS containers have a tendency to leak coffee or to leak the oil and/or fat components in food substances carried by the containers. That is, the coffee or the oil and/or fat permeate around the fused polystyrene beads and through the wall of the container. With regard to the oil and/or fat components, a stain generally forms on the outer surface of the container.
Several approaches have evolved over the years directed toward the reduction of leakage in containers for retaining liquids and/or pre-cooked foods.
Amberg et al., U.S. Pat. No. 4,036,675 discloses a container made from foamed polystyrene, which is lined on one or both sides with unoriented polyolefin film, preferably polypropylene. The film is secured to the foamed plastic base material using a vinylic polymer or polyamide resin as a heat-sensitive adhesive. The film is coated with a wet adhesive and dried before laminating the film to the foam material.
Sonnenberg U.S. Pat. Nos. 4,703,065 and 4,720,429 disclose thermoplastic polymer foam cups for retaining coffee that are molded from thermoplastic polymer particles whose surfaces are coated with a fluorosurfactant before molding.
Sonnenberg U.S. Pat. No. 4,785,022 discloses a method for enhancing the coffee retention of molded foam cups. The method involves coating the expandable polystyrene particles with various rubber polymers and copolymers, e.g. polybutene, polyisobutylene, isobutylene-butene copolymer and butene-ethylene copolymer.
Arch, et al. U.S. Pat. No. 4,798,749 and U.S. Pat. No. 4,840,759 overcome the problem of coffee leakage by replacing conventional blowing agents such as butanes, n-pentane, hexanes, and the halogenated hydrocarbons with isopentane in the expandable styrene polymer particles.
Ikeda, et al., U.S. Pat. No. 4,698,367 discloses expandable thermoplastic resin particles composed of a fluorinated vinyl polymer and a hydrophilic vinyl polymer that covers or is included on the surface or in the surface layer of the expandable thermoplastic particle. These resin particles are useful for producing package containers for oily or fatty foods.
Sakoda et al., U.S. Pat. No. 6,277,491 B1 discloses coating the surface of the resin beads or incorporating resin beads with a fluorine-containing block copolymer comprising a fluorine-containing vinyl-type polymer segment derived from a fluorine-containing vinyl-type monomer and a lipophilic vinyl-type polymer segment derived from a lipophilic vinyl-type monomer.
Suzuki et al., U.S. Pat. No. 4,206,249 discloses a process for producing a paper container having high impermeability to liquid which comprises spray coating a polymerizable solution containing a pre-polymer onto a wall surface of a paper container and irradiating the coated wall with ultraviolet light to effect the setting of the pre-polymer on the wall surface of the container. This forms a coating, which is impermeable to liquids, such as water, milk, soft drinks, oils, etc.
Breining, et al., U.S. Pat. No. 6,416,829 B2 discloses a heat insulating paper cup where the body member is coated on its outside surface with a foamed low density polyethylene, and on its inside surface with an unfoamed modified low density polyethylene.
Japan Unexamined Patent Publication JP2002338725A (Kaneka Corporation) discloses the use of a volatile foaming agent containing 30 to 60% by weight based on the foaming agent of isopentane. The expandable polystyrene resin particles are covered with zinc stearate ranging between 0.2 to 0.5 parts by weight based on 100 parts by weight of resin particle. The amount of foaming agent ranges from 0 to 5.5% by weight of the resin particles.
None of the thermoplastic resin particles of the prior art for making foam containers are coated or covered with a liquid polyethylene glycol and then with one or more components selected from the group consisting of polyolefin wax, e.g. polyethylene wax; a metal salt of higher fatty acids, e.g. zinc stearate; polyethylene glycol; and a fatty bisamide or fatty amide, e.g. ethylene bis-stearamide, for reducing or eliminating the penetration of liquid and/or oil and/or fat components of food items such as coffee, noodles, soups, sauces, stews, meats, and the like through the foam containers.